Depth profiling of nitrogen in silicon wafers by secondary ion mass spectrometry

Trace analysis of nitrogen in silicon wafers was performed using SIMS on N-14 implanted samples. The detection limits for nitrogen were defined as the constant level measured in the tail of (42)(N+Si)(-) depth profiles. The effects of primary ion current and raster scan size on the detection limits were studied. Next, considerable intensity was observed in the negative bias region of the 42(N+Si)(-) energy spectra. These were thought to correspond to ions created by post-ionization of sputtered neutral atoms by primary ion impact, or by delayed-ionization near the sample surface. The potential of using the gas-phase ionization of sputtered neutral atoms to improve the detection limits was then evaluated. In addition,there are mass interference ions (42)(C+Si)(-) for (42)(N+Si)(-). It was found that (42)(N+Si)(-) has a lower energy distribution compared to (42)(C+Si)(-). Under optimal analytical conditions using gas-phase ions and avoiding interference from (42)(C+Si)(-) by the energy filtering technique,the detection limit of 4x10(15) atoms/cm(3) for nitrogen was achieved.